This invention relates to a new process for joining two nickel containing alloy members together by fusion welding, particularly the joining of fuel assembly grids and ducts composed of nickel-bearing stainless steel.
The construction of fuel assemblies for sodium cooled fast breeder reactors requires the joining of spacer grid members to a duct member. This joining is usually done by a well-known gas tungsten arc welding process (GTAW) in which the two members are joined at a point where the metal of each is locally melted. The members are fused together upon solidification of the metal at the weld location to form a common weld nugget.
The melting and solidification process offers opportunity for the microstructure of the metal to change at the site of the weld. Of particular concern is the possible formation in the weld nugget of phases of the alloy which do not include significant amounts of ferrite, a lack which is known to cause poor weld characteristics.
A fuel assembly duct member is a long six-sided tube with wall thickness typically 0.040 inches. The grid member (0.015 inches thick) must be located within the duct at locations of difficult accessibility. The grid member is usually clamped to the duct member, and the welding electrode is applied to the outside wall of the duct member. Thus, the welding process requires the welding electrode to melt entirely through the thickest work piece (the duct member) and partially into or completely through the thinnest work piece (the grid member) to achieve a suitable weld nugget.
Recently, the designs for the duct and grid members specify alloys containing higher amounts of nickel. Attempts to weld this material using the GTAW process result in poor fusion, cracking, and insufficient weld strength. The problem is due to the insufficient formation or absence of phases of ferrite content in the weld nugget.
Consequently, it is desired to provide a welding process which can be used to accomplish the welding of nickel-bearing steel alloy members which do not normally exhibit formation of ferrite phases in the microstructure of the weld nugget.